1. Field of the Invention
The present invention relates to a process for forming a silica film by selective deposition onto a substrate from a liquid composition and a composition for use in this process. The process is simple and easy in handling the composition and can be applied to variety of substrates. The obtained silica films can be used as various insulating films, protecting films and so on.
2. Description of the Related Art
The generally known and used process for forming a silica film for use as an insulating film or a protecting film is that a composition obtained by hydrolysis of alkoxysilane in the presence of an acidic catalyst is coated on a substrate, followed by drying and heat treating the coated composition to form a silica film (for example, Japanese Unexamined Patent Publication (Kokai) No. 56-38472). However, this process has problems in coverage of the coating, appearance of cracks due to drying shrinkage, adhesion to substrates and so on.
Recently, formation of a silica film by deposition from a liquid composition has been attempted to solve the above problems. For example, (i) a substrate is immersed in an aqueous solution of silicofluoric acid in which silicon dioxide has been dissolved to a supersaturation level, to deposit a silica film on the substrate in a liquid phase (for example, Japanese Unexamined Patent Publication (Kokai) No. 57-196744), (ii) alkoxysilane having a hydrophobic group is hydrolyzed to form a silica film on a substrate (for example, Japanese Unexamined Patent Publication (Kokai) No. 7-196342), and (iii) fine silica particles are dispersed in ethanol, tetraethoxysilane is added, and a combination of water, ammonia and ethanol is added, by which silica particles grow (Kagaku Kogaku Ronbunshu (Reports on Chemical Engineering), Vol. 21, No. 5, pp 879-885, 1995), can be mentioned.
However, these methods have the following problems. In the method (i), the solution contains fluoric acid, which makes the handling difficult and limits the substrate due to its corrosive action. In the method (ii), the deposited film contains carbon and fluoride and heat treatment at a high temperature is required to remove these components. It can also mentioned that silicic acid having no hydrophobic group cannot be used. In the method (iii), two pots are used including one for a substrate, tetraethoxysilane and ethanol and the other for water, ammonia and ethanol. This is because if one pot containing all components is prepared and a substrate is then immersed in that solution, a large amount of precipitation is generated soon after the solution is prepared before the immersion of the substrate, which makes the solution inappropriate for deposition of a silica film on a substrate. On the other hand, in the two pot system, an operation of terminating deposition such as dilution of the solution with a large amount of alcohol is required. Accordingly, a continuous process using the same composition is difficult, efficiencies of using starting materials and solvents are low, and the productivity is low, so that it is not suitable in production in industrial scale. The alkali suggested therein is only ammonia and capability of deposition with other alkalis was not confirmed.